Filesystem-DAX is incompatible with 'longterm' page pinning. Withoutpage cache indirection a DAX mapping maps filesystem blocks directly.This means that the filesystem must not modify a file's block map whileany page in a mapping is pinned. In order to prevent the situation ofuserspace holding of filesystem operations indefinitely, disallow'longterm' Filesystem-DAX mappings.

RDMA has the same conflict and the plan there is to add a 'with lease'mechanism to allow the kernel to notify userspace that the mapping isbeing torn down for block-map maintenance. Perhaps something similar canbe put in place for vfio.

Note that xfs and ext4 still report:

"DAX enabled. Warning: EXPERIMENTAL, use at your own risk"

...at mount time, and resolving the dax-dma-vs-truncate problem is oneof the last hurdles to remove that designation.